The present invention relates to an ISDN remote switching unit for accommodating analog and digital lines and, more particularly, to an ISDN remote switching unit comprising a traffic concentrator switch which is disposed remote from the host exchange station.
The conventional telephone network is constructed mainly for the telephone service, wherein the analogue exchange, such as A-type exchange and cross-bar exchange, is usually used. The so-called non-telephone terminals with slow transmission speed, such as data terminal and facsimile terminal, can be connected to the analogue exchange in addition to the usual various telephone terminals. However, the non-telephone terminals with high transmission speed cannot be connected to it. Recently it is strongly requested to provide the connection of the high speed non-telephone terminals with low cost in the field of, for example, the utilization of the data base by means of the personal computer.
Therefore, the integrated service digital network (ISDN) is proposed, wherein the conventional analogue telephone terminal, various high-speed data terminal, video terminal and digital telephone can be connected together. In the end of 1984, the CCITT (International Telegraph and Telephone Consultative Committee) has recommended the basic access method (CCITT I series recommendation) as a standard for information exchange by way of the digital-analogue conversion between the terminal and host exchange.
In this I series recommendation, the basic access system denoted as 2B+D system with transmission speed 144 Kb/s is proposed, in which the speech signal and the data are transmitted by two B channels with 64 Kb/s speed and the control information is transmitted by a D channel with 16 Kb/s.
The basic construction of the ISDN digital exchange is composed of a line system for exchanging subscriber lines, a signal processing system for transmitting/receiving the signal to/from the subscriber and the interstation signal to/from the confronted exchange, a control system for controlling the connection of the calls and management of the exchange, and a maintenance system for monitoring and testing the exchange. Although the analogue exchange exchanges the analogue signals, such as speech signals, as it is, the digital exchange converts the analogue signal into the digital signal by way of the pulse code modulation (PCM), and then exchanges the digital signals through communication paths composed PG,5 of the electronic components, such as LSI.
In general, the subscribers for the ISDN digital exchange are not necessarily concentrated in some area. For example, the relatively small number of them may be distributed in a certain area or they may sparsely be scattered in a large area. Therefore, the employment of the digital exchange for these areas is inefficient.
Accordingly, it is preferable that the traffic concentrator switch is separated from the host digital exchange and disposed in the remote area and that its exchange operation is controlled by the distant host exchange. This is called "ISDN remote station control system".
Now, the conventional remote station used by the above ISDN remote switching unit will be explained with reference to FIGS. 1 and 2, which show the remote switching units 105 for analogue subscriber and 120 for digital subscriber, respectively.
Referring to FIG. 1, the remote switching unit 105 for analogue subscriber is composed of the line circuits 107a and 107b for analogue subscribers, a traffic concentrator switch 108, a remote intra-station controller 109 and a terminator 111. The line circuits 107a and 107b comprise the so-called BORSHCT function, where "B" stands for the battery feed, "O" for the over voltage, "R" for the ringing, "S" for supervision, "H" for the hybrid, "C" for the codec and "T" for the testing. The conventional analogue subscriber lines 106a and 106b are terminated by the line circuits 107a and 107b, respectively. The various signals from the analogue subscriber are multiplexed together with the speech signals so as to compose a PCM signal through a semi-fixed path of the traffic concentrator switch 108 and finally transmitted from the terminator 111 to the host station 113 composed of a digital exchange.
On the other hand, as is shown in FIG. 2, the conventional remote switching unit 120 for the digital subscriber is composed of the ISDN line circuits 117a and 117b, a remote control processor 118, a traffic concentrator switch 108 and a terminator 111.
The transmission lines for the ISDN subscribers, such as the digital telephone sets 114a and 114b, and video terminals 115a and 115b, are terminated through the network terminators (NT) 116a and 116b by the line circuits 117a and 117b, respectively. According to the D channel protocol, the layer 3 messages in the D channel subscriber line signal (described in detail in the following) are multiplexed by the processor 118, and then transmitted through the terminators 111 and 112 to the host station 113. Furthermore, the processor 118 constitutes the transmission path of the channel B in the concentrator switch 108 depending on the message content.
As is described above, if both the conventional analogue subscriber line and the ISDN digital subscriber line should be installed together in an ISDN remote switching unit, it is necessary to include both the remote station unit for the analogue subscriber shown in FIG. 1 and that for the digital subscriber shown in FIG. 2. Namely, there is a problem that both the analogue and digital subscribers can not share a remote switching unit.